DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 125 



out farther laterally : it is connected with the origin of the middle 

 germ-layer. 



In a still later stage of development (fig. 86), at the beginning of 

 the second day of incubation, the first fundament of the central 

 nervous system makes its appearance in the anterior portion of the 

 germ-disc. Over the head-process there arise at some distance from 

 each other the two medullary folds (J.), which are continuous with 

 each other at their anterior ends, and which bound the broad medul- 

 lary furrow (me) ; posteriorly they become less prominent, and they 

 here embrace between them the anterior end of the primitive streak 

 (pr). Medullary furrow (me) and primitive groove (pr) must not 

 be confounded with each other, as occurred in the earlier days of 

 embryology ; they are two entirely distinct and dissimilar structures, 

 which exist at the same time, and independently of each other, as- 

 fig. 86 shows. 



Primitive streak and primitive groove are preserved for a long 

 time without undergoing important changes (fig. 87 pr). They 

 always occupy the posterior end of the embryonic body, which is 

 characterised by its slightly differentiated condition even in stages 

 when the development of the separate organs of the body is already 

 in full progress. On the contrary, the embryonic territory lying in 

 front of it, which is so small at the time of the appearance of the 

 head-process, becomes greatly elongated and, at the same time, 

 differentiated into the separate organs of the body. This process 

 of differentiation begins in front, and proceeds posteriorly toward 

 the primitive groove, just as in Amphioxus and the Amphibia. 

 The margins of the medullary folds come into contact with each 

 other and begin to fuse, forming the neural tube (hb l , hb 2 , hb 3 , 

 m/), the fusion progressing from the head- toward the tail-end. 

 There are also to be recognised now in the interior of the body, 

 at either side of the neural tube, the protovertebrse or primitive 

 segments (us), which we shall investigate more minutely further 

 on. The number of these is constantly increased by the growth 

 which is taking place at the tail-end. 



When a large number of primitive segments has arisen, the 

 primitive groove begins on surface-views to disappear ; for it is sur- 

 rounded by the medullary folds, and inasmuch as these fuse here as 

 well as elsewhere, it is enclosed in the terminal part of the neural 

 tube. A notable condition, and one of great importance for the 

 interpretation of the primitive groove, has been discovered at this 

 stage in the embryos of several species of Birds by GASSER, BRAUN, 



